In U.S. Pat. No. 3,369,626, there is disclosed an acoustic borehole logging technique and system wherein the walls of a borehole are scanned periodically with acoustic energy for obtaining information of interest. In one embodiment, a single transducer which acts both as a transmitter and receiver is rotated in the borehole and periodically actuated to produce acoustic pulses which are applied to the borehole wall. Reflected energy is detected by the transducer between acoustic pulses and converted into receiver signals which are employed to intensity modulate the electron beam of an oscilloscope which is swept across the screen of the oscilloscope once for each rotation of the transducer. Successive traces are produced representative of the borehole wall anomalies or fractures as sensed by the rotating transducer. Each successive trace is photographed by suitable means for the production of a two-dimension, flat record of the surface of the borehole wall.
In one embodiment, the modulating signals have amplitudes dependent upon the energy of reflected acoustic pulses detected whereby the resulting picture has tones of white, gray, and black. The tones of the picture primarily represent the type of reflecting surface, i.e., weak or strong, or the absence of a reflecting surface if fractures are present. Such pictures, hence, are very useful in locating and defining fractures in the formations.
Borehole logging tools generally employed in carrying out such an acoustic logging technique have employed windows in the periphery of the logging tool in juxtaposition with the acoustic transducer and through which acoustic energy is transmitted and received by the transducer. This window preferably consists of a material that is substantially transparent to the transmission of acoustic energy. For example, U.S. Pat. No. 3,550,075 describes such a window as being a rubber boot. U.S. Pat. No. 3,378,097 describes such a window as being of a flexible, neoprene material or a hard, acoustically opaque material. U.S. Pat. No. 3,434,563 describes such a window as being of a 0.05" hycar rubber sleeve. One of the biggest problems is designing this window has been in providing for maximum signal transmission and signal-to-noise ratio. The present invention is, therefore, directed to solving this problem by providing for a new and improved window which serves to so maximize such signal transmission and signal-to-noise ratio.